1. Field of the Invention
The present invention relates to a chip; and more particularly to a light-emitting diode chip with high light extraction efficiency.
2. Description of the Related Art
Please refer to FIG. 1, which shows a conventional light-emitting diode chip 1. FIG. 1 includes a substrate 11, an epitaxial-layer structure 12 on the substrate 11 and an electrode unit 13 constituted of an N-type electrode 131 and a P-type electrode 132.
As an example, the epitaxial-layer structure 12 is formed of GaN-based material and has an N-type first cladding layer 121, an active layer 122 formed on the first cladding layer 121 and a P-type second cladding layer 123. The first cladding layer 121 and the second cladding layer 123 is opposite to each other and form a carrier injector relative to the active layer 122. As such, when power supply is provided to the epitaxial-layer structure 12, electrons and holes would be recombined in the active layer 122, and then release energy in a form of light.
The N-type electrode 131 and P-type electrode 132, for example, are formed of Au, Ni, Pt, Ag, Al, etc. and/or their alloy. The N-type electrode 131 is disposed on and forms ohmic contact with the first cladding layer 121 of the epitaxial-layer structure 12. The P-type electrode 132 is disposed on and forms ohmic contact with the second cladding layer 123 such that the N-type electrode 131 and P-type electrode 132 provide electric power to the epitaxial-layer structure 12.
When electric energy is supplied to the N-type electrode 131 and P-type electrode 132, current spreads and flows through the epitaxial-layer structure 12, and electrons and holes are injected into the active layer 122, recombining with each other and then releasing energy in the form of light.
The refractive index of the GaN-based material is about 2.6, and the refractive index of its surrounding material, which generally is air, is 1, or the surrounding is a transparent encapsulating material used for packaging and having a refractive index of 1.4. The top surface 124 of the second cladding layer 123 of the epitaxial-layer structure 12 of the light-emitting diode chip 1 is a flat surface. Partial light generated from the epitaxial-layer structure 12, due to their propagation direction, would follow Snell's law and would not escape the epitaxial-layer structure 12. As a consequence, the light extraction of the light-emitting diode chip 1 is not good.
Please refer to FIG. 2, there is much literature and patents that propose to roughen the top surface 124′ of the light-emitting diode chip 1′ to make the light impinging on the rough top surface 124′ have various possible incident angles relative to the rough top surface 124′. The possibility of light escaping the epitaxial-layer structure 12′ is thus increased, and the light extraction efficiency is improved.
Nevertheless, the light generated from the epitaxial-layer structure 12′ does not entirely propagate toward the top surface 124′. The light propagating toward the substrate 11′ faces similar situation as that at the top surface and cannot escape the epitaxial layer 12′ to enter the surrounding. Thus, the light extraction is still low.
Some literature proposes to form a reflective mirror layer, which is connected to the epitaxial-layer structure 12′, capable of reflecting light. Hopefully, the light propagating toward the substrate 11′ can be reflected toward the top surface 124′ to improve the possibility of light generated from the epitaxtial-layer structure 12′ to escape the epitaxial structure and enter the surrounding. However, the light propagating toward the substrate 11′ would be confined in the epitaxial-layer structure 12′ due to their propagation directions and cause total internal reflection within the epitaxial-layer structure 12′. Furthermore, the light can be absorbed by the active layer. The reflective mirror layer on the substrate 11′ cannot substantially improve the light extraction of the light-emitting diode chip.
It is intended to improve the structure of the light-emitting diode chips 1, 1′ to increase the light extraction and brightness.